1. Field of the Invention
The present invention generally relates to a safety device for preventing a trouble caused by an abnormal overheating of an electric circuit and more particularly, to a solder fuse, as a safety device, which can be easily formed with the same and normal solder as used for connecting electrical parts, has high reliability as a fuse, and can reduce manufacturing cost.
2. Description of the Related Art
For example, a voltage-dependent varistor is used as a conventional safety device for an electric circuit as proposed in Japanese U.M. Registration Application Laid-open No.1-129744.
In this safety device for an electric circuit, leads of the voltage-dependent varistor are bent in a right angle and are soldered to respective electrodes of first and second circuit patterns which are standoff-insulated each other on the circuit board, whereby the first and second circuit patterns are electrically connected.
In the above structure, the solder fixing the leads melts and then the voltage-dependent varistor falls due to its own weight when the ambient temperature of the first or second circuit pattern or of the voltage-dependent varistor has risen high, whereby the circuit between the first and second circuit patterns is broken. This prevents a trouble caused by an abnormal overheating of the electric circuit.
With respect to the above-described conventional safety device for an electric circuit, however, the voltage-dependent varistor is required and further the varistor requires to be held when the varistor is soldered to a circuit board, thereby consuming time and cost.
In order to solve this kind of problems, a number of safety devices for an electric circuit, in which a solder itself is used as a fuse, have been proposed.
Such a conventional safety device for an electric circuit is described with reference to FIGS. 3 and 4.
FIGS. 3A to 3C are plan views showing a conventional safety device for an electric circuit, which safety device utilizes a solder fuse. And, sectional views of FIGS. 3A to 3C are shown in FIGS. 4A to 4C, respectively.
As shown in FIGS. 3A and 4A, a first circuit pattern 110 and a second circuit pattern 120 are formed on a circuit board 100, and electrodes 111,121 of the respective first and second circuit patterns 110,120 are standoff-insulated each other by arranging a gap 100a therebetween. And, as shown in FIGS. 3B and 4B, a solder fuse portion 101 is formed with a solder 30 bridging the electrodes 111,121.
According to such a structure, when the ambient temperature of the first or second circuit pattern 110,120 or of the solder 30 has risen high, the solder 30 completely melts.
And then, as shown in FIGS. 3C and 4C, the melted solder 30 on the gap 100a is shed by the circuit board having no affinity with the solder 30 and separates on the electrodes 111,121 by surface tension.
Consequently, the circuit between the first and second circuit patterns 110,120 is broken, thereby preventing a trouble caused by an abnormal overheating of the electric circuit.
With respect to the above conventional solder fuse, however, when the solder fuse portion 101 is formed, the solder 30 shall be almost completely melted and is deposited over each of the electrodes 111,121.
When the solder fuse portion 101 is formed, however, since the solder 30 is apt to be separated on each of the electrodes 111,121, formation of the solder fuse portion 101 is difficult, thereby resulting in a low yield rate.
This is caused by form of the gap 100a between the electrodes 111,121, that is, since the gap 100a continues in an arrow direction shown in FIG. 3A, the melted solder 30 on the gap 100a is shed.
However, since the characteristics that the melted solder 30 is apt to be separated onto each of the electrodes 111,121 can be utilized as a fuse, it is necessary that the melted solder 30 does not separate onto the electrodes 111,121 only when the solder fuse portion 101 is formed.
In order to solve the above problem, Japanese Patent Application Laid-open No.4-56028 proposes a safety device for an electric circuit wherein a solder fuse portion is formed with a porous solder layer having cavities and bridging electrodes of respective first and second circuit patterns, which electrodes are standoff-insulated each other on a circuit board.
With respect to the above conventional safety device having such a structure, however, a solder shall be melted incompletely so as to form the porous solder layer, thereby consuming time and cost, compared with formation of a solder fuse portion with the normal solder 30.